Integrated muffler, manifold, and catalyst devices for vehicles having an internal combustion engine are desirable because of the decreased space and weight requirements and the associated cost benefits. They accomplish the goal of reducing emissions, suppressing noise, and directing exhaust flow in a single package. Conventional systems use a cavity for creating a manifold for interfacing to the exhaust ports, a single expansion chamber for expanding the exhaust gas, a concentrating volume for converging the flow into a laminar flow catalyst, and a second concentrating volume for further converging the flow. The single expansion chamber reduces low frequency noise, while the effect of the convergent regions and laminar flow catalyst reduce high frequency noise. Having the catalyst in close proximity to the exhaust ports decrease the heat energy lost from the exhaust gas, thereby decreasing catalyst temperature light-off times and increasing emission control. Such a system is disclosed in U.S. Pat. No. 5,351,483.
The inventors herein have recognized numerous disadvantages with the above approaches. One disadvantage is that the exhaust flow restriction created by the combined manifold, muffler, and catalyst is less than optimal due to the sudden expansion experienced by the exhaust gas when entering the expansion chamber, thereby causing available engine horsepower to decrease from optimal. Another disadvantage is that the large resonator, or expansion, volume needed to suppress low frequency noise increases thermal mass, thereby increasing catalyst light-off time despite the close coupled location of the catalyst. An increase in catalyst light-off time is unsatisfactory because of the corresponding decreased emissions reduction.